METHOD OF TREATING DIARSENIC TRIOXIDE

Abstract

 To provide a method of generating, with good reproducibility and ease and without complicated operations, scorodite which satisfies the elution standard (in accordance with Notification of No.13 of Japanese Environment Agency) and which has good filterbility and stability for processing arsenic contained in a diarsenic trioxide form. A method of processing diarsenic trioxide, including: a leaching step of adding water to diarsenic trioxide to produce slurry, heating the slurry, and leaching arsenic while adding an oxidant to obtain leachate; a deoxidization step of removing the oxidant so as to obtain an adjusted solution; and a crystallizing step of converting arsenic in the adjusted solution to scorodite crystal.

Description

Technical Field

[0001] The present invention relates to a method of processing diarsenic trioxide (As₂O₃) and heavy metal; of extracting arsenic from diarsenic trioxide, and crystallizing it into a scorodite crystal which is a stable arsenic compound.

Background

[0002] There are documents on stabilization of a compound that contains arsenic.
Patent Document 1 describes a method of producing scorodite directed to arsenic that is contained in smelter soot.

[0003] Patent Document 2 relates to a method of leaching arsenic sulfide. It describes adding alkali while blowing air to slurry that contains arsenic sulfide, and leaching arsenic while maintaining the pH to 5 to 8.

[0004] Non-Patent Document 1 reports solubility products of iron arsenate, calcium arsenate and magnesium arsenate. According to the document, while calcium arsenate and magnesium arsenate are stable only in an alkali region, iron arsenate is stable in a neutral to acidic region. The document reports the minimum solubility 20 mg/l at pH 3.2.

[0005] Non-Patent Document 2 discloses solubility of iron arsenate and scorodite. The document indicates that solubility of arsenic from scorodite in a weak acidic region is lower than solubility of arsenic from amorphous iron arsenate by two digits, which discloses that scorodite is a stable arsenic compound.

[0006] Non-Patent Document 3 describes a method of producing scorodite directed to arsenic that is contained in wastewater from sulfuric acid plants or smelters.

[0007] [Patent document 1] Japanese Patent Application Laid Open No. 2005-161123
[Patent document 2] Japanese Patent Publication No. 61-24329
[Non-patent document 1] [Non-patent Document 1] Tadahisa Nishimura, Kazumitsu Tozawa, "Report of Research Institute of Mineral Dressing and Metallurgy, Tohoku University" No. 764, Vol. 34, Separate Print No. 1, 1978, June
[Non-patent document 2] E. Krause and V. A. Ettel, "Solubilities and Stabilities of Ferric Arsenate Compounds" Hydrometallurgy, 22, 311-337, (1989)
[Non-patent document 3] Dimitrios Filippou and George P.Demopoulos, "Arsenic Immobilization by Controlled Scorodite Precipitation" JOM Dec., 52-55, (1997)

Disclosure of the Invention

Problem to be solved by the Invention

[0008] Recently, global environment on non-ferrous smelting for securing mineral raw materials is very harsh. Particularly, the demand is very tight in the filed of cupper smelting where oligopoly by major non-ferrous companies is advancing and new large consumer countries such as developing country have emerged.
Under such circumstances, the individual countries are tightening and mandating more and more regulations in the environmental field. The present inventors have thought that the industry will be lead by mines and smelters that can co-exist with the environment.

[0009] Here, examples of pollutions concerned in non-ferrous smelting are air pollution caused by SO₂ gas, and soil and wastewater pollutions caused by arsenic. Particularly, since the content of arsenic in copper ore is expected to increase in the future, implementation of full-scale measures will be further required than ever.
Conventionally, coastal non-ferrous smelters in Japan have been conducting their operations without problems by using clean concentrates as treated materials. However, since the content of arsenic in copper ore is expected to increase, the inventors thought that an art will be necessary of taking arsenic as smelting intermediates to the outside of the system, stabilizing it in a certain form, and controlling and storing it.

[0010] Accordingly, the present inventors investigated the aforementioned documents.
However, no Patent documents or Non-Patent documents describe a method of processing diarsenic trioxide, which extracts arsenic from diarsenic trioxide or a non-ferrous smelting intermediate containing diarsenic trioxide, and converts it into a stable arsenic compound.

[0011] The present invention has been made under such circumstance. An object of the present invention is to provide a method of processing diarsenic trioxide; which extracts arsenic from diarsenic trioxide, and processes it to a scorodite crystal with good filterbility and stability.

Means for Solving Problems

[0012] The present inventors were dedicated to the research to resolve the aforementioned problem. As a result, they obtained a totally new discovery. They discovered that arsenic can be recovered as scorodite with good filterbility and stability: by implementing a leaching step of adding water and an oxidant to diarsenic trioxide to leach arsenic in leachate; next, implementing a solution adjusting step of removing the oxidant remaining in the leachate so as to obtain an adjusted solution; and further implementing a crystallizing step of converting arsenic in the adjusted solution to a scorodite crystal. In addition, the present inventors discovered that an oxidation reaction of oxidizing trivalent arsenic to pentavalent arsenic in a short period of time can be performed by blowing an oxidized gas into an aqueous solution containing the trivalent arsenic while heating the aqueous solution containing the trivalent arsenic in the presence of the three types of substances that are copper sulfide, copper ions, and copper pentavalent arsenic compounds as catalysts. Moreover, the present inventors confirmed that 99% or more of the trivalent arsenic is oxidized to a pentavalent form at the stop of this oxidation reaction, and have thus achieved the present invention.

[0013] That is, the first means to resolve the aforementioned problem is a method of processing diarsenic trioxide, including: a leaching step of adding water to diarsenic trioxide (As₂O₃) to produce slurry, and leaching arsenic while heating the slurry and adding an oxidant to obtain leachate; a deoxidization step of removing an oxidant remaining in the leachate to obtain an adjusted solution; and a crystallizing step of converting arsenic in the adjusted solution to scorodite crystal.

[0014] The second means is the method of processing diarsenic trioxide according to claim 1, wherein the leaching step uses hydrogen peroxide as the oxidant.

[0015] The third means is the method of processing diarsenic trioxide according to the first means, wherein the leachate is brought into contact with the metallic copper and remaining hydrogen is removed in the deoxidization step.

[0016] The fourth means is the method of processing diarsenic trioxide according to any of the first to third means, wherein ferrous (Fe²⁺) salt is added and dissolved in the adjusted solution and this ferrous salt is oxidized, thereby converting arsenic in the adjusted solution to scorodite crystal in the crystallizing step.

[0017] The fifth means is the method of processing diarsenic trioxide according to the fourth means, wherein one of air and oxygen gas or mixed gas thereof is blown to the adjusted solution to oxidize the ferrous salt.

[0018] The sixth means is the method of processing diarsenic trioxide according to any of the first to fifth means, wherein the crystallizing step is performed in a region where the pH is 1 or less.

[0019] The seventh means is the method of processing diarsenic trioxide according to any of the first to sixth means, wherein the crystallizing step is performed at 50°C or higher.

[0020] The eighth means is an arsenic oxidation method, wherein by blowing at least one of air and/or oxygen gas into a solution that contains at least one of diarsenic trioxide (As₂O₃) and/or arsenous acid ions, is heated to 50°C or higher, has a pH of not less than 1 in a neutral region, and comprises copper sulfide, copper ions, and a copper pentavalent arsenic compound, trivalent arsenic in the solution is oxidized to pentavalent arsenic.

[0021] The nineth means is an arsenic oxidation method, wherein by blowing at least one of air and/or oxygen gas into a solution that contains at least one of diarsenic trioxide (As₂O₃) and/or arsenous acid ions, is heated to 50°C or higher, has a pH of not less than 2 in a neutral region, and comprises copper sulfide, trivalent arsenic in the solution is oxidized to pentavalent arsenic, while generating the copper pentavalent arsenic compound by dissolving a portion of the copper sulfide.

[0022] The tenth means is the arsenic oxidation method according to the eighth or nineth means, wherein the pH is not less than 2 when the blowing of at least one of air and/or oxygen gas starts, and less than 2 when the blowing of at least one of air and/or oxygen gas ends.

[0023] The eleventh means is the arsenic oxidation method according to any of the eighth to eleventh means, wherein after the trivalent arsenic in the solution is oxidized to the pentavalent arsenic, the solution produced by pulp is filtered and a filtering residue is recovered, and the filtering residue is used as a substitute for the copper sulfide.

[0024] The twelfth means is the arsenic oxidation method according to any of the eighth to eleventh means, wherein after the trivalent arsenic in the solution is oxidized to the pentavalent arsenic, the solution produced by pulp is neutralized to bring the pH to not less than 3 and thereby crystallize the copper ions in the solution as the copper pentavalent arsenic compound, and then filtering is performed to recover a filtrate and a filtering residue, and the filtering residue is used as a substitute for the copper sulfide.

Effect of the Invention

[0025] According to one of the first to seventh means, arsenic can be extracted from diarsenic trioxide, and processed to scorodite crystal with good filterbility and stability. Also, according to any of the eighth to twelfth means, trivalent arsenic can be oxidized to pentavalent arsenic at an oxidation rate of 99% or more with low operation costs and low equipment costs, by using materials that are easily obtainable in non-ferrous smelters. Furthermore, according to the present invention, the pH of the solution at the stop of the oxidation reaction is not less than 1 and below 2, which is favorable for producing scorodite (FeAsO₄·2H₂O). In this respect, too, the present invention contributes to low operation costs and low equipment costs.

Best Mode for carrying Out the Invention

[0026] As described above, the present invention relates to a method of processing diarsenic trioxide, including: a leaching step of obtaining leachate in which arsenic is leached while adding an oxidant to diarsenic trioxide; a solution adjusting step of removing the oxidant remaining in the leachate so as to obtain an adjusted solution; and a crystallizing step of converting arsenic in the adjusted solution to scorodite crystal. The present invention also relates to a method of oxidizing trivalent arsenic to pentavalent arsenic at an oxidation rate of 99% or more with low operation costs and low equipment costs.
Hereinafter, a first embodiment will be explained in details on 1. Diarsenic trioxide; 2. Leaching step; 3. Deoxidization step; 4. Crystallizing step; and Example 1 in order while referring to a flowchart shown in FIG. 1.
Next, as the second embodiment, a method of oxidizing trivalent arsenic to pentavalent arsenic at an oxidation rate of 99% or more with low operation costs and low equipment costs will be explained on the 1. Processing object; 2. Oxidation reaction of trivalent arsenic; 3. pH of the trivalent arsenic at the beginning of the oxidation reaction; 4. pH of the trivalent arsenic at the stop of the oxidation reaction; and Examples 2 to 6 and Comparative Examples 1 to 5 will be described in order in detail while referring to the flowchart shown in Fig. 2, and further the 5. Trivalent arsenic oxidation reaction model conceived by the present inventors will be described.

1. Diarsenic trioxide

[0027] Diarsenic trioxide (1) according to the present invention is recovered as an intermediate product in industries such as a non-ferrous smelter industry or the like. In addition, use of the present processing method is not limited in smelters that discharge arsenic to the outside of the system as a non-ferrous smelting intermediate containing diarsenic trioxide in an existing smelting step. It is also effective for smelters in which arsenic is stored as diarsenic trioxide.

2. Leaching step

[0028] The leaching step (2) according to the present invention will now be explained.
In the leaching step (2) according to the present invention, water is added to diarsenic trioxide in solid power (1) to produce slurry, the slurry is heated, and arsenic is leached while adding an oxidant.
In the case where the diarsenic trioxide (1) contains high moisture content, the amount of water to be added should be reduced. Note that the water may be industrial water, intermediate water in the treatment process or the like. The oxidant should be preferably added after the slurry is produced, because this will make the reaction more stable.
Normally, a small amount of the diarsenic trioxide dissolves in water as trivalent arsenic as shown below (Equation 1). However, since solubility of trivalent arsenic is small, it is difficult to obtain a highly-concentrated arsenic solution. In the meantime, FIG. 2 showed solubility of diarsenic trioxide in water.

        As₂O₃ (diarsenic trioxide) + H₂O = 2HAsO₂     (Equation 1)

FIG. 2 is a graph with the vertical axis showing values of solubility of diarsenic trioxide and arsenic, and the horizontal axis showing the temperature. In the graph, solubility of arsenious acid in water is plotted as ■, and solubility or arsenic in the aforementioned diarsenic trioxide is plotted as • at each temperature.

[0029] Meanwhile, in the present invention, a higher arsenic concentration at the start of the reaction is more preferable to increase productivity of producing scorodite (6) in the crystallizing step (5) described later. Specifically, the arsenic concentration 40g/l or higher is required in the leachate (3).
In this case, it is understood from FIG. 2 that the solution temperature need always be kept at 80°C or higher to obtain a trivalent arsenic solution having a concentration as high as 40g/l or higher. However, in the actual operation, an operation such as filtering or the like is required, and therefore, the solution temperature drops and arsenious acid is crystallized, which causes clog of filter fabric. Accordingly, a problem that the production is inoperable or the like occurs.

[0030] As a result of the investigation to resolve the problem, the present inventors herein have arrived a breakthrough mechanism of leaching trivalent arsenic in the diarsenic trioxide (1) as pentavalent arsenic having a greater solubility.
For reference, solubility of arsenic pentoxide (As₂O₅), which is pentavalent arsenic, is very large. (For example, the solubility is reported in p. 167 in Chemistry Handbook, Basic II, Revision 3, edited by the Chemical Society of Japan. It says that the mass of As₂O₅ contained in 100 go of a saturated As₂O₅ solution is 39.7 g at 20°C.)
Accordingly, the present inventors have arrived an idea that use of the mechanism of leaching trivalent arsenic in diarsenic trioxide (1) as pentavalent arsenic having high solubility enables obtaining leachate (3) having an arsenic concentration as high as 40g/l that is required in the crystallizing step (5) easily.

[0031] Specifically, the method is to add water to diarsenic trioxide to produce slurry, heat it, and perform leaching while adding an oxidant to it. However, since the oxidation reaction from trivalent arsenic to pentavalent arsenic becomes is the rate-limiting reaction, trivalent arsenic in the solution is concentrated highly, whereby arsenic cannot be leached.
Therefore, the present inventors have arrived an idea of using hydrogen peroxide as a preferable example of the oxidant. Use of hydrogen peroxide as the oxidant enables the oxidation reaction from trivalent arsenic to pentavalent arsenic to proceed in a short period of time. In addition, substantially all amounts of arsenic can be leached as pentavalent arsenic under the leaching condition described later.
Note that, the present inventors consider that the comprehensive dissolution reaction of diarsenic trioxide in water as pentavalent arsenic in the case where hydrogen peroxide is used as the oxidant is expressed in (Equation 2) and (Equation 3).

        As₂O₃ + H₂O + 2H₂O₂ = 2H₂AsO₄^- + 2H⁺     (Equation 2)

        As₂O₃ + H₂O + 2H₂O₂ = 2H₃AsO₄     (Equation 3)

Note that it is naturally conceivable Equation 2 and Equation 3, in place of H₂O₂, another oxidant having oxidizing power which is almost as strong as, or stronger than that of H₂O₂ can also be used. Such oxidant is also within the scope of the present invention.
A residue (7) that is generated may be put into a copper smelting (8) step.

[0032] Next, the reaction condition in the leaching step reaction will be described.

<Dissolving method and time>

[0033] Diarsenic trioxide and water in certain amounts are blended to produce a pulp. The temperature of the pulp is increased to 50°C or higher, at which hydrogen peroxide is added. 10 to 15 minutes is enough for adding hydrogen peroxide. When the addition is completed, the solution temperature will have reached approx. 80°C as described later. Then, stirring is maintained for 60 minutes or more in this state. The reaction can be judged as completed at the time when the redox potential of the solution becomes 450 mV (Vs; Ag/AgCl) or less at 80°C.

(The amount of hydrogen peroxide to be added)

[0034] The amount of hydrogen peroxide to be added may be the theoretical amount (i.e., 1 time equivalent) necessary for oxidation reaction of trivalent arsenic, when it is assumed that all arsenic contained in the pulp is trivalent arsenic.
The amount of hydrogen peroxide to be added is sometimes determined with use of the redox potential as a guideline. In this case, the amount is accpeptable as long as the redox potential after the addition is 500 mV or higher (Vs; Ag/AgCl) at 80 °C.
Note that, commonly used hydrogen peroxide of concentration of 30 to 35% may be used.

(Reaction Temperature)

[0035] The solution temperature should be preferably 50°C or higher. This temperature is preferable because this promotes dissolution of diarsenic trioxide in water and the oxidation reaction from dissolved trivalent arsenic to pentavalent arsenic.
Note that, in the case where hydrogen peroxide is used as the oxidant, the oxidation reaction from trivalent arsenic to pentavalent arsenic caused by addition of hydrogen peroxide is the exothermic reaction. Accordingly, for example, in the case where a solution of an arsenic concentration as high as approx. 50g/l is to be prepared, when the solution temperature when addition of hydrogen peroxide is started is 60°C, the solution temperature when addition thereof in the certain amount will be approx. 80°C, although exothermic condition from the equipment is different.

3. Deoxidization step

[0036] The deoxidization step (4) is the step of removing the oxidant remaining in the leachate (3).
The oxidant that remains after the reaction oxidizes a part of ferrous (Fe²⁺) which is added in the crystallizing step (5) which is the next step. Accordingly, it is preferable to remove the oxidant to accurately control the ferrous ion concentration.
A method of adding metallic colloid such as gold, silver, or the like and dissolving it is also conceivable in order to remove hydrogen peroxide that remains in the leachate (3). However, taking into consideration its handling property or generation of loss, the method is not appropriate in the actual operation.
Accordingly, the present inventors have achieved the mechanism of removing the remaining oxidant (for example, hydrogen peroxide) by consuming the oxidant rather than by degrading it. Specifically, the oxidant is brought into contact with the oxidized agent (for example, metallic cupper) and removed by being consumed as shown in (Equation 4).

        Cu⁰ + H₂O₂ + H₂SO₄ = CuSO₄ + 2H₂O     (Equation 4)

The reaction temperature is preferably 40°C or higher to complete the reaction. In addition, since the reaction accompanies increase in the pH, the reaction can be judged as completed at the time when the pH shows a constant value.

4. Crystallizing step

[0037] The crystallizing step (5) is a step of crystallizing pentavalent arsenic in the adjusted solution which is obtained in the deoxidization step (4) into scorodite (6).
With productivity of scorodite (6) being taken into consideration, the adjusted solution obtained after the deoxidization step (4) is preferably a highly-concentrated solution of arsenic concentration 20g/l or higher, and more preferably of 30g/l or higher.
First, ferrous (Fe²⁺) salt is added and dissolved into the adjusted solution, and sulfuric acid (H₂SO₄) is added at the room temperature so as to adjust the pH to pH = 1. Among a variety of ferrous salt compounds, ferrous sulfate is preferable from the perspective of corrosion resistance of the equipment and easiness of availability.
The additive amount of ferrous salt is 1 time equivalent or more of the number of moles of arsenic to be processed as iron purity, and preferably 1.5 times equivalent or more.

[0038] When ferrous salt is added and the pH is adjusted, the temperature of the adjusted solution is increased to a predetermined reaction temperature. In this case, as long as the reaction temperature is 50°C or higher, it is possible to precipitate scorodite (6). However, a higher reaction temperature is more preferable from the perspective of increasing the grain size of scorodite. Note that, the reaction temperature is preferably set to 90 to 100°C from the perspective of making the reaction under ambient atmosphere possible.

[0039] When the temperature of the adjusted solution has reached the predetermined reaction temperature, blowing of one of air and oxygen gas, or a mixture gas thereof is started, and the solution is stirred strongly to create the gas-liquid mixing state. While the predetermined reaction temperature is maintained, an oxidation reaction at a high temperature is caused to proceed.
The oxidation reaction at a high temperature is thought to proceed as shown in (Equation 5) to (Equation 10) in approximately 2 to 3 hours.

<The first half of the reaction >

[0040] 

        2FeSO₄ + 1/2O₂ + H₂SO₄ = Fe₂ (SO₄)₃ + H₂O     (Equation 5)

        2H₃AsO₄ + Fe₂ (SO₄)₃ + 4H₂O = 2FeAsO₄·2H₂O + 3H₂SO₄     (Equation 6)

<Whole reaction equation ((Equation 5) + (Equation 6) is shown in (Equation 7) as follows.>

[0041] 

        2H₃AsO₄ + 2FeSO₄ + 1/2O₂ + 3H₂O = 2FeAsO₄·2H₂O + 2H₂SO₄     (Equation 7)

<Latter half of the reaction where As concentration has dropped>

[0042] 

        2FeSO₄ + 1/2O₂ + H₂SO₄ = Fe₂ (SO₄)₃ + H₂O     (Equation 8)

        2/3H₃AsO₄ + 1/3Fe₂ (SO₄)₃ + 4/3H₂O = 2/3FeAsO₄·2H₂O + H₂SO₄     (Equation 9)

<Whole reaction Equation ((Equation 8) + (Equation 9)) is shown in (Equation 10) as follows.>

[0043] 

        2/3H₃AsO₄ + 2FeSO₄ + 1/2O₂ + 4/3H₂O = 2/3FeAsO₄·2H₂O + 2/3Fe₂ (SO₄)₃     (Equation 10)

[0044] Although affected by the oxidation method, the pH, the arsenic concentration and the Fe concentration sharply dropped within 2 to 3 hours after the oxidation reaction at a high temperature started. At this stage, the indicated redox potential of the solution is 400 mV or higher (Vs; Ag/AgCl) at 95°C. In addition, 90% or more of the arsenic contained is a crystal of scorodite (6). After three hours have passed after starting the oxidation reaction at a high temperature or thereafter, arsenic that remains in the solution decreases by only a small amount, while the pH or the redox potential rarely changes. Note that, the oxidation reaction at a high temperature is preferably continued for 5 to 7 hours to end the oxidation reaction at a high temperature in the complete equilibrium condition.

[0045] The aforementioned crystallizing step (5) according to the present invention allows easy reaction operation and can convert the contained arsenic into scorodite (6) crystal without the need of adjusting pH in the middle of the reaction.
A filtrate (9) that is generated may be treated by a wastewater treatment step (10).
The obtained scorodite (6) crystal is excellent in sedimentability and filterability, and achieves volume reduction with adhesive moisture approximately 10% after filtering and high arsenic grade as much as 30%. In addition, the crystal is excellent in elution resistance and stability. Therefore, it is possible to remove arsenic in a stable form from the smelting step and store it.

[Examples]

[0046] Examples will now been shown to describe the present invention more specifically.

(Example 1)

<Leaching>

[0047] 58.0 g of diarsenic trioxide as an intermediate product of non-ferrous smelting with the grade as shown in Table 1 was introduced in a 1 L beaker (with 4 baffle plates). Further, 850 cc of pure water was added and the solution was heated to 60°C while being stirred.
Subsequently, addition of hydrogen peroxide was started. In this case, hydrogen peroxide having a concentration as high as 30% was used.
When it is assumed that all arsenic contained in the arsenious acid was trivalent arsenic, the amount of hydrogen peroxide to be added is 1-time equivalent of the theoretical amount required for oxidizing it to pentavalent arsenic. Specifically, the amount of hydrogen peroxide having a concentration as high as 30% is 65.7 g.
The time for adding hydrogen peroxide was set as 12 minutes. The temperature at the stop of addition was adjusted to 80°C.
After addition of hydrogen peroxide is ended, stirring was maintained at 80°C for 90 minutes to complete the reaction.
Subsequently, the leachate was recovered by suction filtration with use of filter paper No. 5C.
The grade of the recovered leachate is shown in Table 2. Note that arsenic is pentavalent arsenic.
The transitions of the temperature, the pH and the redox potential during the reaction are shown in Table 3. Note that the redox potential is Ag/AgCl reference value.

[0048] 

[Table 1]

As	S	Cu	Bi	Cd	Fe	Pb	Sb	Zn
(%)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)
74.88	1,538	27	784	2	963	82	172	13

[0049] 

[Table 2]

As	S	Cu	Bi	Cd	Fe	Pb	Sb	Zn
(%)	(mg/l)	(mg/l)	(mg/l)	(mg/l)	(mg/l)	(mg/l)	(mg/l)	(mg/l)
48.47	89	<1	<1	<1	43	4	10	<1

[0050] 

[Table 3]

Elapsed time (minutes)	0	12	42	57	87	102
Reaction course	Immediately before addition	At the stop of addition	While the reaction is maintained			At the stop of leach
Temperature (°C)	60.2	80.5	80.1	80.0	80.6	80.3
pH	2.69	1.32	1.29	1.30	1.29	1.30
Redox potential (mV)	270	538	517	441	435	432

<Deoxidation>

[0051] A powdery metallic copper of reagent grade 1 was prepared as deoxidant.
2.5 g of a powdery metallic copper was added to the obtained leachate, and the deoxidization step was performed at the reaction temperature 40 °C to obtain an adjusted solution.
Table 4 shows the transition of the reaction. Note that the redox potential is Ag/AgCl reference value.

[0052] 

[Table 4]

Elapsed time (minutes)	0	1	5	10	15
Reaction course	Start of reaction	Addition of Cu powder	While the reaction is maintained		At the stop of reaction
Temperature (°C)	39.4	41.2	40.5	41.0	40.8
pH	1.16	1.17	1.19	1.19	1.19
Redox potential (mV)	424	341	259	236	206

[0053] Note that the concentration of each element was the same as those shown in Table 2, except that Cu concentration increased to 121 mg/l.

<Crystallization>

[0054] The obtained adjusted solution was diluted with pure water, and the arsenic concentration was adjusted to 45 g/l. The solution 800 cc was transferred to a 2 L beaker, and its pH was adjusted to 1.15 by adding 95% sulfuric acid.
Ferrous (Fe²⁺) salt containing iron content of 1.5 times equivalent to the number of moles of the arsenic that is contained was added to the diluted solution. Specifically, 200g each of ferrous sulfate (FeSO₄·7H₂O) of reagent grade 1 was introduced in the beaker, and dissolved. Next, the pH was adjusted to pH = 1.0 at 30°C by adding 95% sulfuric acid.
After pH was adjusted, the solution was heated to the temperature of 95°C, and subsequently, gas blowing was started in oxygen gas at 950 cc/minute using a glass tube from the bottom of the beaker. The oxidation reaction at a high temperature was caused to occur under strong stirring and in the gas-liquid mixing state for 7 hours to produce scorodite. The conversion rate of arsenic contained in the adjusted solution to scorodite was 96.9 %. The property and the elution value of the generated scorodite are shown in Table 5. Note that the elution value was measured in compliance with Notification of No.13 of Japanese Environment Agency.

[0055] 

[Table 5]

As sedimentation rate (note 1)	Scorodite
	Moisture content	Composition (%)		Elution value (mg/l) (note2)
(%)	(%)	As	Fe	As	Pb	Cd	Hg	Se
96.9	9.0	31.3	25.1	<0.01	<0.01	<0.01	<0.005	<0.1

(Note 1) As sedimentation rate: Conversion rate of As in the solution to scorodite (Note 2) As elution value: In compliance with Notification of No.13 of Japanese Environment Agency

Second Embodiment

[0056] According to the research of the present inventors, the above oxidation method using hydrogen peroxide (H₂O₂) achieves approximately 100% oxidation of trivalent arsenic by accelerating the trivalent arsenic oxidation speed and causing the reaction at a high solution temperature. However, hydrogen peroxide is an expensive agent.

[0057] On the other hand, the oxidation method using ozone (O₃) achieves approximately 100% oxidation of trivalent arsenic in a short period of time, irrespective of solution temperature. However, this oxidation method has the following problems.
Ozone generating equipment itself requires high costs. Furthermore, ozone has strong oxidizing power, so that the specification of peripheral apparatuses needs to be upgraded. This results in extremely high costs for the system as a whole.
Because ozone is hazardous to humans, an ancillary facility for collecting and detoxifying ozone that is released to the atmosphere without reaction is necessary.
Ozone is easy to dissolve in water than oxygen gas, and the solution after reaction has a peculiar pungent odor. To resolve this problem, a process of removing dissolved ozone in a subsequent step is necessary.

[0058] Meanwhile, it became clear that the method of adding powdery metallic copper or the like as a catalyst has the following problems.

1) In the case where the solution to be treated has a low arsenic concentration (for example, approximately 3 g/L), the oxidation rate of arsenic is approximately 100%. However, in the case where the solution to be treated has a high arsenic concentration (for example, 60 to 70 g/L), the oxidation rate of arsenic drops to approximately 79%.

2) When metallic copper (Cu°) changes to copper ions (Cu²⁺), the change of trivalent arsenic to pentavalent arsenic is affected. In addition, at the time of this change, at least the number of moles of metallic copper equivalent to trivalent arsenic is required. Furthermore, the same effects as metallic copper are confirmed even in a poor water soluble copper compound (Cu₂O, CuS). As a result, a large amount of agent (copper source) is necessary when processing arsenous acid being a trivalent arsenic compound.

3) As explained in the above 2), this method uses a large amount of copper source when processing arsenous acid (trivalent arsenic). As a result, copper ions as many as several tens of g/L remains in the solution after the reaction. Therefore, a process of recovering copper from the solution after the reaction is necessary, which causes an increase in copper recovery costs.

4) This reaction is conducted in the acidic solution (for example, the pH is 0 and the FA (free acid) value is 130 g/L), so that a large amount of acid content remains in the solution after the reaction. In order to produce a pentavalent arsenic compound based on the solution after the reaction, a large amount of alkali is necessary. This is an inevitable problem as this method requires dissolving powdery metallic copper and/or a poor water-soluble copper compound,

[0059] Hereinafter, the second embodiment for carrying out the present invention will be described. The 1. Processing object; 2. Oxidation reaction of trivalent arsenic; 3. pH of the trivalent arsenic at the beginning of the oxidation reaction of, 4. pH of trivalent arsenic at the stop of the oxidation reaction; Examples 2 to 6, and Comparative Examples 1 to 5 will be described in order in detail while referring to the flowchart shown in Fig. 3, and further the 5. Trivalent arsenic oxidation reaction mechanism model conceived by the present inventors will be described.

[0060] According to this embodiment, trivalent arsenic can be oxidized to pentavalent arsenic at an oxidation rate of 99% or more with low operation costs and low equipment costs, by using materials that are easily obtainable in non-ferrous smelters.

1. Processing Object

[0061] This embodiment is an optimum processing method for producing a highly concentrated arsenic solution.
In other words, according to this embodiment, trivalent arsenic of low solubility can be easily oxidized to pentavalent arsenic of high solubility. Therefore, by using diarsenic trioxide <1> which is solid as the trivalent arsenic source, the diarsenic trioxide dissolves simultaneously with the oxidation of trivalent arsenic to pentavalent arsenic, which ensures the timely supply of trivalent arsenic. As a result, a pentavalent arsenic solution of a concentration as high as several tens of g/L, that is, a concentrated arsenic acid solution can be easily produced.

2. Oxidation reaction of trivalent arsenic

[0062] In order to derive this embodiment relating to the oxidation step <4>, the present inventors investigated the step of oxidizing trivalent arsenic by oxygen gas, using copper as an oxidation catalyst for arsenic.
Several points that are subject to the investigation are given below.

[0063] 

1) Using only copper ions as an oxidation catalyst (corresponding to Comparative Examples 5 and 6 described later).

2) Using only copper sulfide as an oxidation catalyst (corresponding to Comparative Example 7 described later).

3) Using the two types of oxidation catalysts of copper sulfide and copper ions together (corresponding to Comparative Example 8 described later).

4) Using the three types of oxidation catalysts of copper sulfide, copper ions, and a copper pentavalent arsenic compound together (corresponding to Examples 5 to 9 described later).

[0064] As a result of the above investigation, the oxidation catalyst effects of copper were observed in all of 1) to 4). However, 4) was found to have dramatic improvements in the oxidation catalyst effects of copper when compared with 1) to 3), in terms of oxidation speed and oxidation rate.
Based on this discovery, it was determined that copper sulfide, copper ions, and a copper pentavalent arsenic compound (copper arsenate) are used together as oxidation catalysts.
Hereinafter, (a) copper sulfide source, (b) copper ion source, (c) copper pentavalent arsenic compound (copper arsenate), (d) reaction temperature, and (e) blowing gas type and blowing amount will be described in detail.

(a) Copper sulfide source

[0065] Copper sulfide solid, copper sulfide powder, and the like can be used as the copper sulfide source <2>. Furthermore, the powdery state is preferable from the perspective of ensuring reactivity. In addition, copper sulfide can be mainly classified into the two compositions of CuS and Cu₂S (there is also Cu₉S₅ being a composition in which a portion of copper in crystal lattice is defective). In this embodiment, any of them is effective, and a mixture of them is also possible. Moreover, the copper sulfide source is preferably as pure copper sulfide as possible (copper sulfide of high purity with minimum impurities). This is because contamination with As₂S₃, ZnS, PbS, CdS, and the like can be avoided by using copper sulfide of high purity.
If contaminated with As₂S₃, ZnS, PbS, CdS, and the like occurs, the following reactions occur. As a result, the supply of copper ions necessary for the oxidation reaction of trivalent arsenic is hindered (Equation 11 to 141).
Furthermore, regarding As₂S₃, that is, arsenic sulfide, even when copper ions are added consciously, the following reaction occurs, which not only makes the maintenance of an optimum copper ion concentration difficult, but also causes hydrogen ion (H⁺) evolution reaction. When hydrogen ions (H⁺) are generated, the pH of the reaction system drops. This makes it difficult to maintain the oxidation reaction of trivalent arsenic according to the present invention, and makes it difficult to oxidize trivalent arsenic.

[0066] 

        Cu²⁺ + 1/3As₂S₃ + 4/3H₂O = CuS + 2/3HAsO₂ + 2H⁺ (Equation 11)

        Cu²⁺ + ZnS = CuS + Zn²⁺ (Equation 12)

        Cu²⁺ + PbS = CuS + Pb²⁺ (Equation 13)

        Cu²⁺ + CdS = CuS + Cd²⁺ (Equation 14)

[0067] Consider the case where copper sulfide recovered as smelting intermediates is used as the copper sulfide source <2>. The recovered copper sulfide contains substantial amounts of the aforementioned As₂S₃, ZnS, PbS, CdS, and the like. Therefore, it is not preferable to use the copper sulfide recovered as smelting intermediates directly as the copper sulfide source <2>. However, the recovered copper sulfide can be used if the aforementioned sulfides are removed beforehand by decomposition reaction or the like to thereby increase the purity as copper sulfide.

[0068] In copper smelters, copper sulfide of high purity suitable for the present invention can be easily produced according to the following method.

(1) Electrolytic copper is dissolved (Cu = 10 to 30 g/L) by aeration while heating under sulfite acidic conditions (FA (free acid) = 50 to 300 g/L), to obtain a copper solution.

(2) The obtained copper solution is reacted with a sulfidizing agent such as NaSH or H₂S at a temperature of 50°C or more, to recover copper sulfide.

(3) The recovered copper sulfide is washed with water to remove adhered acid content.
The copper sulfide after the water cleaning has little impurities, and is suitable for the present invention in any of the dry condition and the wet condition.

(b) Copper ion source

[0069] A substance that becomes copper ions in the solution to be treated can be used as the copper ion source <3>. For example, copper sulfide is preferable, as it is solid at ordinary temperatures, but dissolves into water and immediately becomes copper ions. Though metallic copper or metallic copper powder can also be used, it is necessary to wait for the dissolution until they are ionized.

(c) Copper pentavalent arsenic compound (copper arsenate)

[0070] Copper arsenate is available as the copper pentavalent arsenic compound according to the present invention. Copper arsenate has a solubility product comparable to iron arsenate (FeAsO₄), and is a pentavalent arsenic compound that is easily formed in the weakly acidic to neutral region.
In this embodiment, copper sulfide is added to the solution containing trivalent arsenic with the initial pH value being set to 2 or more, and the oxidation reaction is started. Thus, the oxidation of the trivalent arsenic to pentavalent arsenic and the supply of copper ions by the dissolution of the copper sulfide occur simultaneously on the surface of the added copper sulfide, and therefore the generation of copper arsenate is though to occur instantaneously. When the reaction is complete, the solution is naturally transferred to the weakly acidic region. By this time, however, the pentavalent arsenic and the copper ions are both concentrated to the order of g/L. Due to this concentration, the generative capacity of the copper arsenate will not decrease.
At this point, unless the pH of the solution sinks below 1 into the acidic state, the forming capacity of the copper arsenate will not decrease significantly. Accordingly, it is preferable to control the pH.

(d) Reaction temperature

[0071] The oxidation of arsenic is preferably performed at a higher solution temperature. Specifically, a temperature of 50°C or more is required for the progress of the oxidation of arsenic. The solution is heated <5> to 70 to 90°C and preferably about 80°C, in consideration of real operation and based on the premise such as the material of the reaction tank and the filtering operation after the reaction.

(e) Blowing gas type and blowing amount

[0072] The oxidation reaction of trivalent arsenic is possible even when the blowing gas <6> is air. However, when oxygen gas or a gas mixture of air and oxygen gas is used as the blowing gas <6>, the oxidation speed is maintained even in the range where the arsenic concentration in the solution is low, and the blowing (gas) capacity decreases. As a result, heat loss associated with this is reduced, and the maintenance of the reaction temperature becomes easier. Therefore, it is preferable to use oxygen gas or a gas mixture of oxygen gas and air as the blowing gas <6>, in terms of the oxidation speed and the reaction temperature maintenance.

[0073] Regarding the blowing amount per unit time of the blowing gas <6>, its optimum value changes depending on the gas-liquid mixing state in the reaction tank. For example, by using a microscopic bubble generation apparatus and the like, the oxidation efficiency can be further improved, and the blowing amount can be reduced.
Therefore, at the time of real operation, it is important to find the optimum value in consideration of the gas-liquid mixing state, the oxygen gas blowing method, and the like.

3. pH of trivalent arsenic at the beginning of the oxidation reaction

[0074] A basic equation of the oxidation reaction of trivalent arsenic according to the present invention is thought to be the following.

        AS₂O₃ + H₂O = 2HAsO₂     (Equation 15)

Reaction in which diarsenic trioxide dissolves in water as arsenous acid (trivalent arsenic).

        2HAsO₂ + O₂ + 2H₂O = 2H₂AsO₄^- + 2H⁺     (Equation 16)

Reaction in which arsenous acid (trivalent arsenic) oxides.

        2HAsO₂ + O₂ + 2H₂O = 2H₃AsO₄     (Equation 17)

Reaction in which arsenous acid (trivalent arsenic) oxides.

[0075] As in the Examples described later, in the case of the concentrated solution whose arsenous acid concentration at the time of complete arsenic dissolution is 40 g/L or more, the solubility of arsenous acid is small, and therefore diarsenic trioxide does not dissolve totally in the initial stage.
In the case of the concentrated arsenic solution, simultaneously with the oxidation of arsenous acid to arsenate of high solubility according to (Equation 16) and (Equation 17) and the decrease of the arsenous acid concentration, the reaction in which arsenous acid is added into the system is thought to proceed. In other words, the solid diarsenic trioxide is thought to dissolve while being suspended in the initial stage of the reaction (Equation 15).

[0076] At this point, the oxidation of arsenous acid to arsenate is thought to be in accordance with (Equation 16) and (Equation 17).
In the oxidation reaction of arsenous acid to arsenate, the behavior in which the pH of the solution rapidly decreases to about 2 is shown in initial 30 minutes. From this behavior, it can be estimated that the oxidation mainly proceeds according to (Equation 16) in the neutral region where the pH is 2 or more. Meanwhile, the decrease of the pH becomes gradual in the subsequent 30 minutes, and so it can be estimated that the reaction mainly proceeds according to (Equation 17).
In view of the above, it can be understood that the efficient oxidation of trivalent arsenic and the control of the pH at the stop of the reaction to the weakly acidic state according to the present invention can be achieved by setting the pH at the beginning of the oxidation reaction (when the air and/or oxygen gas blowing starts) to 2 or more.

4. pH of trivalent arsenate at the stop of the oxidation reaction

[0077] In this embodiment according to the present invention, the pH of trivalent arsenate at the stop of the oxidation reaction (when the air and/or oxygen gas blowing stops) was below 2 and more specifically about 1.8 in all cases, as shown by the results of Examples 5 to 9 described later.
This pH of about 1.8 is a preferable pH for producing a pentavalent arsenic compound (the acid concentration is at an adequate level). This is because the optimum pH range for producing iron arsenate which is a pentavalent arsenic compound is pH = 3.5 to 4.5, and so the neutralizing agent consumed for neutralizing acid content can be reduced.
On the other hand, in the production of scorodite (FeAsO₄·2H₂O), the pentavalent arsenic solution whose pH is about 1 is used as the stock solution, and therefore the pH can be adjusted by adding a small amount of inverse neutralizing agent (for example, sulfuric acid). Furthermore, the pH at the stop of the reaction is preferably not less than 1 and below 2, though the details will be described in Example 6 below.

[0078] The pH at the stop of the trivalent arsenic oxidation reaction (when the air and/or oxigen blowing stops) being below 2 and specifically about 1.8 is thought to be derived from the above (Equation 15) to (Equation 17).
First, according to (Equation 15), diarsenic trioxide is dissolved in water as arsenous acid (trivalent arsenic). Furthermore, this is not limited to the case where the starting row material is the solid diarsenic trioxide, but also applies to the case of the aqueous solution in which arsenic trioxide has already been dissolved as arsenous acid (therefore, the present invention is thought to be applicable to ordinary drainage treatment).

[0079] The product obtained in the above oxidation step <4> is separated in the filtering <7> into the filtrate <8> and the filtrand <9>. In the filtering <7>, an ordinary filtering method such as filter press can be applied. This is because, though a copper pentavalent arsenic compound is generated in teh above oxidation step <4>, there is no problem of filterbility such as increased viscosity.

[0080] The obtained filtrate <7> is an arsenate solution having a pH of about 1.8 as mentioned above. Since the pH of about 1.8 is preferable for producing pentavalent arsenic compounds, a pentavalen arsenic compound can be produced from the filtrate <7> with low costs and high productivity.
On the other hand, the filtrand <9> is a mixture of copper sulfide and a copper pendavalent arsenic compound, and accordingly can be repeatedly used as it is as an oxidation catalyst. When repeatedly using this, the catalyst effect can be expected to increase by newly adding copper sulfide of an amount equivalent to partially dissolved copper sulfide.

5. Trivalent arsenic oxidation reaction mechanism model

[0081] The ternary catalyst made up of copper sulfide, copper ions, and a copper pentavalent arsenic compound according to the present invention has both a high oxidation rate and a high oxidation speed. The oxidation catalyst effects exhibited by this ternary catalyst is thought to be derived from the battery-like reaction caused by the contact of each type of ionson the copper sulfide surface.

[0082] For example, consider the model of the oxidation reaction mechanism using the region of about pH = 2 as an example.
First, substituting the trivalent arsenic oxidation to electrode reactions yields (Equation 18) showing the anodic reaction and (Equation 19) showing the cathodic reaction.

        As₂O₃ + 5H₂O = 2H₃OAsO₄ + 4H⁺ + 4e^-     (Equation 18)

        4H⁺ + O₂ + 4e^- = 2H₂O     (Equation 19)

In other words, the oxidation reaction of trivalent arsenic proceeds as shown in (Equation 18), but it is necessary to maintain electrical neutralization in order to have the reaction proceed. Therefore, the reactivity depends on the progress of the cathodic reaction shown in (Equation 19) which proceeds on the copper sulfide surface. Due to this, it is thought to be important to secure the copper sulfide surface which always has a high activation level.

[0083] Which is to say, in the present reaction model system, copper ions coexist and also the reaction occurs in the weakly acidic pH region, and therefore the crystallizing reaction of the copper sulfide compound as shown in (Equation 20) is thought to occur on the copper sulfide surface.

        Cu²⁺ + H₃AsO₄ + H₂O = CuHAsO₄·H₂O + 2H⁺     (Equation 20)

According to (Equation 20), it can be considered that hydrogen ions (H⁺) are added to the copper sulfide surface and the reactions shown in (Equation 21) and (Equation 22) proceed simultaneously.

        CuS + 2H⁺ + 1/2O₂ = Cu²⁺ + S° + H₂O     (Equation 21)

        CuS + H⁺ + 2O₂ = Cu²⁺ + HSO₄^-     (Equation 22)

[0084] At this time, the copper arsenate compound is formed on the copper sulfide surface, so that the oxygen gas supply becomes insufficient and the S° (monatomic sulfur) generating reaction as shown in (Equation 21) is likely to proceed. Further, with the progress of (Equation 21) and (Equation 22), it is estimated that the Cu ion concentration increases locally and also the hydrogen ion (H⁺) concentration decreases. At this location, the copper sulfide generating reaction shown in (Equation 23) is thought to proceed simultaneously with the above (Equation 21) and (Equation 22).

        Cu²⁺ + 4/3S° + 4/3H₂O = CuS + 1/3HSO₄^- +7/3H⁺     (Equation 23)

[0085] (Equation 23) shows the crystallization of CuS which is copper sulfide, and indicates that the CuS crystallization is ensured on the copper sulfide surface as the newly-formed surface of high activity.

[0086] Furthermore, the hydrogen ions (H⁺) generated in (Equation 23) are supplied to the reactions shown in (Equation 21) and (Equation 22), and also consumed in the dissolution reaction of the copper arsenate compound (the inverse reaction of (Equation 20)). As a result, the addition of copper ions to the copper sulfide surface and the dispersion of arsenic acid (H₃AsO₄) to the periphery are thought to proceed.
Note, in the condition of pH = 0 shown in Comparative Example 5 below, basically the reaction shown in (Equation 20) does not proceed and the reaction shown in (Equation 23) does not proceed easily, and so it is interpreted that the oxidation efficiency drops significantly.

Examples

(Example 2)

[0087] Diarsenic trioxide of reagent grade (the grade is shown in Table 6) and copper sulfide of reagent grade (the grade is shown in Table 7) were prepared.
As described above, copper sulfide can be mainly classified into the two forms of CuS and Cu₂S, and there is also a composition Cu₉S₅ in which a portion of copper in crystal lattice is defective. Any of these forms is usable, and a mixture of these forms is applicable too.
The results of X-ray diffraction of copper sulfide used in this Example are shown in Fig. 4. Note, in Fig. 4, the peak of CuS is plotted as Δ, the peak of Cu₂S is plotted as *, and the peak of Cu₉S₅ is plotted as ◆. From the results of X-ray diffraction, the copper sulfide used in this Example is thought to be the mixture of CuS, Cu₂S, and Cu₉S₅.

[0088] 

Table 6

arsenic	sulfur	copper	zinc	lead	cadmium
(%)	(ppm)	(ppm)	(ppm)	(ppm)	(ppm)
74.8	1,303	27	11	60	2

Table 7

copper	sulfur	zinc	lead	cadmium
(%)	(%)	(ppm)	(ppm)	(ppm)
71.2	26.1	29	2	1

[0089] A 1 L beaker was used as the reaction vessel, a 2-stage turbine blade and 4 baffle plates of 700 rpm were used as the mixture device, and the gas blowing was conducted by blowing in oxygen gas using a glass tube from the bottom of the beaker (the oxidation was performed in a gas and liquid mixture in vigorous mixing).

[0090] 50 g of diarsenic trioxide and 48 g of copper sulfide were introduced in the reaction vessel, 800 cc of pure water was added to repulp, and the solution was heated to 80°C. Next, the mixture of the solution was started using the mixture device, and further the blowing of oxygen gas from the bottom of the reaction vessel was started at 400 cc/min, to oxidize trivalent arsenic. Note, the pH of the solution immediately before the oxygen gas blowing start was 3.09 (at 80°C).

[0091] The solution mixture and the oxygen gas blowing were continued for 90 minutes to oxidize the trivalent arsenic. The temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution were measured every 30 minutes. The measurement results are shown in Table 8. Note, the redox potential is Ag/AgCl reference electrode value.

[0092] 

Table 8

Elapsed time (minutes)	30	60	90
Temperature (°C)	79	79	79
pH	2.13	1.88	1.84
Redox potential (mV)	298	327	383
Cu2+(g/L)	1.8	4.0	5.6
Trivalent arsenic (g/L)	29.2	8.3	0.2
Pentavalent arsenic (g/L)	13.9	33.2	40.7
Oxidation rate (%)	32.3	80.0	99.5

[0093] After the oxidation of the trivalent arsenic was continued for 90 minutes, the solution was filtered, the catalyst recovered as the residue was washed with water, and the grade analysis and X-ray diffraction of the catalyst were performed. The grade analysis results and X-ray diffraction results of the catalyst after the reaction are shown in Table 9 and Fig. 5, respectively. In Fig. 5, the peak of Cu is plotted by Δ, and the peak of the copper pentavalent arsenic compound is plotted by o.

[0094] 

Table 9

copper	sulfur	arsenic
(%)	(%)	(%)
54.2	22.6	10.5

[0095] From Table 8, Table 9, and Fig. 5, it can be understood that copper sulfide, copper ions, and a copper pentavalent arsenic compound (copper arsenate) coexist in the reaction system according to Example 2.
Moreover, it can be understood that the oxidation speed and the oxidation rate of the trivalent arsenic are high in Example 2. In particular, it was confirmed that the oxidation rate of 99% or more was already reached at the point of 90 minutes after the oxidation reaction start.

(Example 3)

[0096] The same operations and measurements as in Example 2 were performed except that the amount of copper sulfide introduced in the reaction vessel was 24 g which is one half.
Note, the pH of the solution immediately before the oxygen gas blowing start was 2.96 (at 80°C).
The results of measuring the temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution every 30 minutes are shown in Table 10, and the analysis results of the grade of the catalyst recovered as the residue and washed with water are shown in Table 11.

[0097] 

Table 10

Elapsed time (minutes)	30	60	90	120
Temperature (°C)	79	80	80	80
pH	2.17	1.88	1.80	1.79
Redox potential (mV)	301	317	336	384
Cu 2+ (g/L)	1.1	2.1	3.1	4.5
Trivalent arsenic (g/L)	32.6	21.3	7.4	0.3
Pentavalent arsenic (g/L)	11.4	24.1	38.0	45.6
Oxidation rate (%)	25.9	53.1	83.7	99.4

Table 11

copper	sulfur	arsenic
(%)	(%)	(%)
63.4	29.4	2.3

[0098] In Example 3, the CuS additive amount is reduced by half of Example 2, to examine the effects of this reduction by half.
As a result, the oxidation speed of trivalent arsenic decreased a little when compared with Example 2, but the oxidation capacity was sufficiently maintained, and the oxidation of 99% or more was observed at the point of 120 minutes after the oxidation reaction start. As with Example 2, the oxidation capacity and speed of trivalent arsenic can both be considered favorable for practical use.

(Example 4)

[0099] This Example is similar to Example 2, but further 16 g of copper sulfide of reagent grade (CuSO₄·5H₂O) was introduced into the reaction vessel. The amount of copper sulfide introduced is equivalent to 5 g/L as copper ions. This Example relates to the case of increasing the copper ion concentration than in the initial stage of the reaction.
Note, the pH of the solution immediately before the oxygen gas blowing start was 2.98 (at 80°C).
The results of measuring the temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution every 30 minutes are shown in Table 12.

[0100] In this Example, the oxygen gas blowing was stopped at 120 minutes when the reaction ended. After this, a NaOH solution of concentration 500 g/L was added to neutralize the solution to pH = 3.5, copper ions existing in the solution were crystallized as a pentavalent arsenic compound, and then the filtering operation was performed. Note, the additive amount of the NaOH solution was 40 cc.
The total arsenic concentration in the filtrate obtained as a result of the filtering operation was 29.6 g/L, while the copper concentration was 80 mg/L. Thus, the concentration decrease associated with the formation of the copper arsenate compound was observed.
On the other hand, the residue recovered as a result of the filtering operation was 165 g·wet. Extracting 5 g·wet of this residue and measuring the moisture content produced the results that the moisture content = 59.9%. In addition, 5 g·wet of the residue was washed with water and the grade was analyzed. The analysis results of the grade of the recovered residue are shown in Table 13.

[0101] 

Table 12

Elapsed time (minutes)	30	60	90	120
Temperature (°C)	79	79	80	80
pH	1.84	1.86	1.90	1.79
Redox potential (mV)	299	321	356	386
Cu 2+ (g/L)	6.1	8.0	10.1	10.9
Trivalent arsenic (g/L)	34.7	17.0	0.7	0.2
Pentavalent arsenic (g/L)	7.9	27.9	42.8	41.0
Oxidation rate (%)	18.5	62.2	98.5	99.5

Table 13

copper	sulfur	arsenic
(%)	(%)	(%)
47.5	12.1	19.7

[0102] Example 4 increases the Cu ion concentration than in the initial stage of the reaction in Example 2. From the results of Table 12, it can be understood that the reaction was complete at a high oxidation rate in Example 4, too.
On the other hand, in Example 4, the oxidation speed decreased a little when compared with Example 2. This indicates that the copper ion concentration in the reaction system need not increased more than necessary. It can be judged that the sufficient copper ion concentration in the reaction system is approximately 1 to 5 g/L.

[0103] Furthermore, when using copper sulfide immediately after being produced by the wet sulfidation reaction, this copper sulfide has a behavior of poor solubility. In view of this, when using copper sulfide immediately after being produced by the wet sulfidation reaction, the addition of copper ions to the reaction system is effective.
Moreover, this Example recovers added copper ions as a copper pentavalent arsenic compound by neutralization. The method of recovering copper ions is not limited to the method of recovering as a copper pentavalent arsenic compound, and may instead be a method of adding an agent that reacts with copper ions and forms copper sulfide, such as monatomic sulfur or ZnS.

(Example 5)

[0104] 50 g of diarsenic trioxide of reagent grade was prepared.
The whole residue recovered in Example 4 (except 10 g·wet used for the measurement sample in Example 4) and 50 g of diarsenic trioxide were introduced into the reaction vessel, and 707 cc of pure water was added to repulp, to bring the moisture content in the pulp to be 800 cc. This pulp was heated to 80°C, and then oxygen gas was started to be blown in from the bottom of the reaction vessel at 400 cc/min.
Note, the pH of the solution immediately before the oxygen gas blowing start was 3.03 (at 79°C).

[0105] The results of measuring the temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution every 30 minutes are shown in Table 14.

[0106] 

Table 14

Elapsed time (minutes)	30	60	90
Temperature (°C)	80	80	79
pH	2.20	1.90	1.83
Redox potential (mV)	294	349	382
Cu 2+ (g/L)	2.2	3.2	4.7
Trivalent arsenic (g/L)	24.2	2.4	0.2
Pentavalent arsenic (g/L)	24.4	48.5	52.3
Oxidation rate (%)	50.2	95.3	99.6

[0107] After the reaction for 90 minutes, the oxygen gas blowing was stopped, a NaOH solution of concentration 500 g/L was added to neutralize the solution to pH = 3.0, and then the solution was filtered. Note, the amount of the NaOH solution used was 36 cc.
The total arsenic concentration in the filtrate obtained was 44.8 g/L, while the Cu concentration was 210 mg/L. Thus, the recovery of the arsenic concentration approximately equivalent to the composition concentration was observed.
On the other hand, the residue recovered was 122 g·wet. Extracting 5 g·wet of this residue and measuring the moisture content produced the results that the moisture content = 48.9%. In addition, 5 g·wet of the residue was washed with water and the grade was analyzed. The analysis results of the grade of the catalyst recovered as the residue are shown in Table 15.

[0108] 

Table 15

copper	sulfur	arsemic
(%)	(%)	(%)
44.4	10.6	21.8

[0109] This Example 5 exhibited highest oxidation efficiency and a highest oxidation speed of Examples 2 to 6. Specifically, the oxidation of 95% was already observed at the point of 60 minutes from the reaction, and the oxidation rate of 99.6% which is approximately 100% was observed at the point of 90 minutes from the reaction.
The catalyst according to this Example 5 is the ternate catalyst of copper sulfide, copper ions, and a copper arsenate compound (copper pentavalent arsenic compound), too. The catalyst according to this Example 5 especially has a high content ratio of the copper arsenate compound (copper pentavalent arsenic compound) compare to Example 2 and Example 3. This high content ratio of the copper arsenate compound is thought to contribute to the improved oxidation performance. In other words, this contribution phenomenon demonstrates that the formation and presence of the copper arsenate compound relates to the generation of the newly-formed surface of CuS of high activity.

(Example 6)

[0110] The same operations as in Example 3 were performed except that the pH immediately before the oxygen gas blowing start was adjusted to 1.0 (at 80 °C) by adding concentrated sulfuric acid to the pulp.

[0111] The results of measuring the temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution every 30 minutes are shown in Table 16. Moreover, the catalyst grade after the reaction (washed with water) are shown in Table 17.

[0112] 

Table 16

Elapsed time (minutes)	30	60	90	120
Temperature (°C)	81	79	80	79
pH	1.22	1.15	1.15	1.13
Redox potential (mV)	363	371	375	380
Cu 2+ (g/L)	4.8	5.2	5.7	6.3
Trivalent arsenic (g/L)	33.6	24.4	17.6	12.8
Pentavalent arsenic (g/L)	10.9	21.2	28.2	33.4
Oxidation rate (%)	24.5	46.5	61.6	72.3

Table 17

copper	sulfur	arsemic
(%)	(%)	(%)
66.0	31.1	0.6

[0113] Example 6 is similar to Example 3 in the amount of copper sulfide added, but the pH of the solution immediately before the oxidation start was adjusted to 1.
As a result, the oxidation capacity decreased when compared with Example 3, and the oxidation rate was 72% at the point of 120 minutes. Though the reaction needs to be performed for a long period of time to reach the oxidation rate of 100%, the oxidation capacity itself is sufficient.

[0114] The reason of the above oxidation speed decrease can be attributed to the fact that the coexisting copper sulfide was significantly reduced. Furthermore, when the pH of the solution is 1, the amount of dissolution of copper sulfide increases, so that the amount of copper sulfide recovered without dissolving (amount of recycle) decreases, which is disadvantageous in terms of cost, too.
In view of the above, it is thought to be preferable to start the reaction by setting the pH of the solution to not less than 2 and ending the oxidation reaction with a pH of not less than 1, in terms of ensuring the reactivity and the CuS recovery amount.

(Comparative Example 1)

[0115] The same operation as in Example 2 was performed except that 50 g of diarsenic trioxide of reagent grade alone was introduced in the reaction vessel and 800 cc of pure water was added to repulp.
Note, the pH of the solution immediately before the oxygen gas blowing start was 2.80 (at 80°C).
The temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution were measured every 30 minutes. The measurement results are shown in Table 18.

[0116] 

Table 18

Elapsed time (minutes)	30	60	90
Temperature (°C)	80	79	80
pH	2.71	2.68	2.67
Redox potential (mV)	378	373	370
Cu 2+ (g/L)	<0.1	<0.1	<0.1
Trivalent arsenic (g/L)	42.0	44.0	45.5
Pentavalent arsenic (g/L)	0	0.1	0.4
Oxidation rate (%)	0	0.2	0.9

[0117] In Comparative Example 1, it was observed that the oxidation of trivalent arsenic proceeded little.

(Comparative Example 2)

[0118] The same operation as in Example 2 was performed except that 50 g of diarsenic trioxide of reagent grade and 16 g of copper sulfide of reagent grade (CuSO_4·5H₂O) were introduced in the reaction vessel and 800 cc of pure water was added to repulp.
Note, the pH of the solution immediately before the oxygen gas blowing start was 3.33 (at 80°C).
The temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution were measured every 30 minutes. The measurement results are shown in Table 19.

[0119] 

Table 19

Elapsed time (minutes)	30	60	90
Temperature (°C)	81	79	80
pH	3.22	3.16	3.10
Redox potential (mV)	373	378	382
Cu 2+ (g/L)	5.3	5.5	5.7
Trivalent arsenic (g/L)	40.3	43.6	45.3
Pentavalent arsenic (g/L)	0.5	0.9	1.3
Oxidation rate (%)	1.2	2.0	2.8

[0120] In Comparative Example 2, though the progress of oxidation was observed when compared with Comparative Example 1, but the degree of progress was still small.

(Comparative Example 3)

[0121] The same operation as in Example 2 was performed except that 50 g of diarsenic trioxide of reagent grade and 32 g of copper sulfide of reagent grade (CuSO_4·5H₂O) (10 g/L as copper ions) were introduced in the reaction vessel and 800 cc of pure water was added to repulp. Note, the pH of the solution immediately before the oxygen gas blowing start was 3.45 (at 80°C).
The temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution were measured every 30 minutes. The measurement results are shown in Table 20.

[0122] 

Table 20

Elapsed time (minutes)	30	60	90
Temperature (°C)	79	81	79
pH	3.29	3.20	3.25
Redox potential (mV)	369	372	378
Cu 2+ (g/L)	10.7	10.6	10.8
Trivalent arsenic (g/L)	39.5	42.5	43.4
Pentavalent arsenic (g/L)	2.5	3.0	3.5
Oxidation rate (%)	6.0	6.6	7.4

In Comparative Example 3, the progress of oxidation was observed as a result of increasing the Cu ion concentration in the solution. However, the degree of progress of oxidation was still small, and further addition of copper ions is thought to be necessary. Hence Comparative Example 3 is not suitable for practical use.

(Comparative Example 4)

[0123] The same operation as in Example 2 was performed except that 50 g of diarsenic trioxide of reagent grade, 48 g of copper sulfide of reagent grade (CuS), and 20 g of sulfur powder were introduced in the reaction vessel and 800 cc of pure water was added to repulp.
Note, the pH of the solution immediately before the oxygen gas blowing start was 2.67 (at 80°C).
The temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution were measured every 30 minutes. The measurement results are shown in Table 21.

[0124] 

Table 21

Elapsed time (minutes)	30	60	90
Temperature (°C)	79	79	81
pH	1.75	1.65	1.63
Redox potential (mV)	340	341	343
Cu 2+ (g/L)	<0.1	<0.1	<0.1
Trivalent arsenic (g/L)	35.2	35.3	35.4
Pentavalent arsenic (g/L)	10.4	10.7	10.9
Oxidation rate (%)	22.8	23.3	23.5

[0125] After the end of the reaction, the solution was filtered, the obtained residue was washed with water, and the grade analysis and X-ray diffraction were performed. The catalyst grade after the reaction (washed with water) is shown in Table 22, and the X-ray diffraction results are shown in Fig. 6.
In Fig. 6, the peak of CuS is plotted by Δ, and the peak of sulfur is plotted buy ■.
In the grade analysis, 0.1% arsenic was detected, but this can be considered to result from the un washed solution adhesion.
From Fig. 6 and Table 22, it can be understood that there is no presence of copper ions and a copper pentavalent arsenic compound in this Comparative Example. 4 to a single catalyst system of copper sulfide.

[0126] 

Table 22

copper	sulfur	arsemic
(%)	(%)	(%)
49.5	50.0	0.1

In this Comparative Example 4, the progress of oxidation was observed. This indicates that single copper sulfide has a higher oxidation capacity as a catalyst than single Cu ions used in Comparative Examples 2 and 3. However, the degree of progress of oxidation is still not appropriate in terms of practical use.

(Comparative Example 5)

[0127] The same operation as in Example 2 was performed except that concentrated sulfuric acid was added to pulp, the pH was adjusted to 0 (at 80°C), and then the oxygen gas blowing was started.
The temperature, pH, redox potential, copper ion amount, trivalent arsenic amount, and pentavalent arsenic amount of the solution were measured every 30 minutes. The measurement results are shown in Table 23.

[0128] 

Table 23

Elapsed time (minutes)	30	60	90	120
Temperature (°C)	80	79	80	80
pH	0.00	0.00	-0.02	-0.04
Redox potential (mV)	411	415	412	411
Cu 2+ (g/L)	9.7	10.8	11.2	11.5
Trivalent arsenic (g/L)	32.7	31.9	32.6	31.6
Pentavalent arsenic (g/L)	1.7	2.8	3.5	4.8
Oxidation rate (%)	4.9	8.0	9.7	13.1

[0129] After the end of the reaction, the solution was filtered, the obtained residue was washed with water, and the grade analysis and X-ray diffraction were performed. The catalyst grade after the reaction (washed with water) is shown in Table 24, and the X-ray diffraction results are shown in Fig. 7. In Fig. 7, the peak of CuS is plotted by Δ, and the peak of diarsenic trioxide is plotted by □.

[0130] 

Table 24

copper	sulfur	arsemic
(%)	(%)	(%)
56.2	28.9	10.6

[0131] In Comparative Example 5, the oxidation of arsenic did not progress, and 10.6% arsenic was detected even in the catalyst after the reaction. Moreover, since diarsenic trioxide was acknowledged from the X-ray diffraction results as shown in Fig. 7, it can be understood that the diarsenic trioxide remained without dissolving even after the oxidation reaction.
This is thought to be because the solubility of diarsenic trioxide decreased since the oxidation reaction was started in the sulfuric acidified solution having a pH of 0, and also because trivalent arsenic eluted into the solution remains without being oxidized to pentavalent arsenic of high solubility and therefore the trivalent arsenic concentration in the solution did not decrease and a portion of diarsenic trioxide remains without dissolving.

[0132] The results of Comparative Example 5 indicate that, when starting the arsenic oxidation reaction under a condition where the pH is 0 which does not allow formation of copper sulfide, the substances that serve as catalysts are the binary system of copper sulfide and copper ions, which results in a significant drop of the oxidation capacity. This demonstrates that the arsenic oxidation reaction according to the present invention is preferably started under a condition where the pH is not less than 1.

Brief Description of Drawings

[0133] 

Fig. 1 is a flowchart showing a method of processing arsenic according to the present invention.

Fig. 2 is a graph showing solubility of diarsenic trioxide in water.

Fig. 3 is a flowchart according to an embodiment (second embodiment) of the present invention.

Fig. 4 shows the X-ray diffraction results of copper sulfide in Example 2.

Fig. 5 shows the X-ray diffraction results of the residue in Example 2.

Fig. 6 shows the X-ray diffraction results of the residue in Comparative Example 4.

Fig. 7 shows the X-ray diffraction results of the residue in Comparative Example 5.

Claims

1. A method of processing diarsenic trioxide, comprising:

a leaching step of adding water to diarsenic trioxide (As₂O₃) to produce slurry, and leaching arsenic while heating the slurry and adding an oxidant to obtain leachate;

a deoxidization step of removing an oxidant remaining in the leachate to obtain an adjusted solution; and

a crystallizing step of converting arsenic in the adjusted solution to scorodite crystal.

2. The method of processing diarsenic trioxide according to claim 1, wherein
hydrogen peroxide is used as the oxidant in the leaching step.

3. The method of processing diarsenic trioxide according to claim 1, wherein
the leachate is brought into contact with the metallic copper and remaining hydrogen is removed in the deoxidization step.

4. The method of processing diarsenic trioxide according to any of claims 1 to 3, wherein
ferrous (Fe²⁺) salt is added and dissolved in the adjusted solution and this ferrous salt is oxidized, thereby converting arsenic in the adjusted solution to scorodite crystal in the crystallizing step.

5. The method of processing diarsenic trioxide according to claim 4, wherein
one of air and oxygen or mixed gas thereof is blown to the adjusted solution to oxidize the ferrous salt.

6. The method of processing diarsenic trioxide according to any of claims 1 to 5, wherein
the crystallizing step is performed in a region where the pH is 1 or less.

7. The method of processing diarsenic trioxide according to any of claims 1 to 6, wherein
the crystallizing step is performed at 50°C or higher.

8. An arsenic oxidation method, wherein
by blowing at least one of air and/or oxygen into a solution that contains at least one of diarsenic trioxide (As₂O₃) and/or arsenous acid ions, is heated to 50°C or higher, has a pH of not less than 1 in a neutral region, and comprises copper sulfide, copper ions, and a copper pentavalent arsenic compound,
trivalent arsenic in the solution is oxidized to pentavalent arsenic.

9. An arsenic oxidation method, wherein
by blowing at least one of air and/or oxygen into a solution that contains at least one of diarsenic trioxide (As₂O₃) and/or arsenous acid ions, is heated to 50°C or higher, has a pH of not less than 2 in a neutral region, and comprises copper sulfide,
trivalent arsenic in the solution is oxidized to pentavalent arsenic, while generating the copper pentavalent arsenic compound by dissolving a portion of the copper sulfide.

10. The arsenic oxidation method according to claim 8 or 9, wherein
the pH is not less than 2 when the blowing of at least one of air and/or oxygen starts, and less than 2 when the blowing of at least one of air and/or oxygen ends.

11. The arsenic oxidation method according to any of claims 8 to 10, wherein
after the trivalent arsenic in the solution is oxidized to the pentavalent arsenic, the solution produced by pulp is filtered and a filtering residue is recovered, and the filtering residue is used as a substitute for the copper sulfide.

12. The arsenic oxidation method according to any of claims 8 to 11, wherein
after the trivalent arsenic in the solution is oxidized to the pentavalent arsenic, the solution produced by pulp is neutralized to bring the pH to not less than 3 and thereby crystallize the copper ions in the solution as the copper pentavalent arsenic compound, and then filtering is performed to recover a filtrate and a filtering residue, and the filtering residue is used as a substitute for the copper sulfide.

Drawing